1. Field of the Invention
The present invention relates to a semiconductor device applied to a high-voltage MOS transistor for controlling a nonvolatile semiconductor storage device, such as a NAND flash memory.
2. Background Art
Nonvolatile semiconductor storage devices include a NAND flash memory. The NAND flash memory has a row decoder circuit that selects from among word lines of memory cells. The row decoder circuit has a structure in which high-voltage MOSFETs (insulated-gate field effect transistors) associated with the word lines are arranged in an array.
In writing to a memory cell transistor, to decode the memory cell transistor to be written, a high program voltage (about 24 V, for example) is applied to the drain of the associated high-voltage MOS transistor.
At the same time, a pass voltage (about 13 V, for example) is applied to the drains of the high-voltage MOSFETs connected to the word lines of the memory cell transistors arranged in the bit line direction in the same block as the memory cell transistor to be written.
The program voltage and the pass voltage are supplied to the gates of the memory cell transistors through the respective word lines, thereby turning on the memory cell transistors.
The voltage applied to the drain of the diffusion layer of the high-voltage MOS transistor associated with the block including the memory cell not to be written is 0 V.
Therefore, in writing to a memory cell transistor, a high voltage is applied to the device isolation insulating film that isolates the high-voltage MOSFETs from each other.
Therefore, for example, if the device isolation insulating film is insufficiently formed, a field inversion can occur below the device isolation insulating film, and thus a current can flow between adjacent high-voltage MOSFETs.
Thus, to ensure the breakdown voltage between the high-voltage MOSFETs, for example, a conventional semiconductor device has a field inversion preventing layer immediately below the device isolation insulating film formed by implantation of about 1×1017/cm3 to 1×1019/cm3 of boron ions (B) (see Japanese Patent Laid-Open No. 2004-297044).
Another conventional semiconductor device has a shielding gate to which 0 V or a negative voltage is applied formed on the device isolation insulating film. In this case, formation of a channel below the device isolation insulating films is prevented, and thus field inversion is prevented (see Japanese Patent Laid-Open No. 2006-59978).
The conventional semiconductor devices described above can ensure the breakdown voltage of the device isolation insulating film between a high-voltage MOSFET to which the program voltage is applied and the adjacent high-voltage MOSFET in writing to a memory cell transistor. That is, the NAND flash memory can normally operate.
However, according to the configurations of the conventional semiconductor devices described above, a sufficient width of the device isolation insulating film needs to be wider (about 2 um, for example) in order to ensure the breakdown voltage against punch through when the potential difference between the diffusion layers (drains) of the adjacent high-voltage MOSFETs.
As a result, there is a problem that miniaturization of conventional nonvolatile semiconductor storage devices is difficult.